AFAIK almost everything was done by Brandon. He had an intern/assistant for a little while, that he delegated some things to, but I think the newsletter was well before that. It was pretty straight forward, usually featured a Thread of the Week that was the most interesting, informative, or helpful. A few other mentions of other things going on, and promotion of one of Brandon's products if he had anything going on at the time.
For a good while, Brandon wrote a forum article almost every week, and the article frequently was the feature of his newsletter. That takes discipline and consistency. I'm sure he made himself write an article weekly so his newletter would have content. The point of the newsletter is kind of moot if there's nothing to say. The content should somewhat create itself through intentional forum content or at least featuring user content. RecordingReview also had a prolific blog feature which was associated with the forum and was used by many members, not just Brandon. He would frequently feature a good blog article by a member.
I remember when I got involved with RR it showed like 86,000 users/members at that point, and it continued to grow. Now, many of those had come and went, but my point is there was a large user base. IRD shows 261 users/members as of today. There have been a grand total of 2 articles written on the forum in it's 8 months of existence.
Brandon was about the most irreverent, poor spelling, bad grammar, insulting SOB you could ever meet. But people liked him. I don't know if he considered himself a natural writer or if he just decided to make himself a writer, but yeah he just got in there and did it and didn't seem to care what anybody thought (maybe he actually did, and the thick skin was a decoy ).
I also remember that members who hadn't been around for awhile would suddenly show up the day the newsletter went out (or the day after). Especially if there was some cool new news. If a mixing contest had been announced, there would be a big RUSH. It was like clockwork. The newsletter reminds people to check in. And Brandon would go in spurts of lots of commenting on lots of threads (in his irreverent spouting way), or not much at all. The forum would be a roller-coaster depending on his participation. It was a Cult of Personality in many ways. It was exciting.